Conventional glass lenses bend light only at its surfaces. At the interface between air and glass, rays of light change direction according to an abrupt change in the index of refraction. By carefully controlling the shape and smoothness of the lens surfaces, these rays can be brought to a focus and form an image. Alternatively, GRIN (GRadient INdex) lenses gradually vary the index of refraction within the lens material and light rays are smoothly and continually redirected towards a point of focus. The internal structure of this index “gradient” dramatically reduces the need for tightly-controlled surface curvatures and results in a simple, compact lens geometry.
GRIN lenses are sometimes utilized for efficient optical coupling to integrated optical devices, especially for coupling between optical fibers and integrated optical waveguides having a high delta n. The alignment of an optical fiber with a GRIN lens and an integrated optical device is complicated because three independent devices must be fixed at a point in space in such a way to provide optimal alignment.